In general, a certain heat exchanger based on aluminum has been widely used which is constructed with a heat exchanger tube formed of aluminum or an aluminum alloy (hereafter referred to as being based on aluminum) and an aluminum-based fin brazed with each other. Further and to gain improved heat exchange efficiency, an extruded flattened tube has been employed as a heat exchanger tube, which extruded flattened tube is derived from an extruded shape based on aluminum.
For the resultant heat exchanger to be proof to corrosion (resistant to corrosion), zinc (Zn) has generally been caused to adhere to the flat heat exchanger tube on its outer surface, followed by diffusion of Zn over the outer surface of such flat tube during brazing with heat and by subsequent formation of a Zn diffusion layer thereover. Additionally, the fin has been prepared from a brazing sheet chosen from among those materials clad with a Zn-containing brazing material, namely of a brazing composition. As a like instance, U.S. Pat. No. 4,831,701 discloses a method in which an aluminum-based fin clad on its surface with a brazing composition derived from Al--Si alloy and flux coated with Zn are applied to and brazed with the flat heat exchanger tube.
Such a brazing sheet for use in fin preparation, however, is costly as compared to a blank fin material not clad with a brazing material. Moreover, because of the cladding on its surface with a brazing material, the brazing sheet leads to rapid abrasion of a fin working roll, thus needing frequent polishing of such working roll. As a further problem, flashes or burrs tend to take place when in working of a louver or the like with eventual quality deterioration of the finished heat exchanger core. Furthermore, by the method of U.S. Pat. No. 4,831,701, process of coating flux with Zn is responsible for increased process step and hence for added production cost and material cost.
In order to improve the corrosion resistance of the flat heat exchanger tube, a method has been employed in which zinc (Zn) is made adherent in advance to an outer surface of such flat tube as by a zincate method or a spray coating of Zn, and a Zn diffusion layer is then formed in the course of brazing so that the flat tube is prevented against corrosion by electrode shielding. However, this method leaves the problem that Zn adhesion is rather tedious and time-consuming.
On the other hand, a method is known in which a blank fin material not clad with a brazing material is put to use and in which a powdered Al--Si alloy derived from aluminum (Al) and silicon (Si) is applied to and brazed with a flat heat exchanger tube. Such known method needs for the powdered Al--Si alloy to be coated in a markedly large amount and hence poses those drawbacks related to cost and assembly. By another known method, a flat heat exchanger tube is prepared from an electrically welded tube clad with a brazing material. For example, JP59086899 discloses a method in which an aluminum-based heat exchanger tube clad on its surface with a brazing composition derived from Al--Si alloy is put to use and in which aluminum-based fin containing Zn applied to and brazed with the flat heat exchanger tube. In this and last method, however, an insert is required to be disposed internally of such tube when rendered multi-channel, and this is responsible for increased process step and hence for added production cost and material cost.